Database change capture with transaction-consistent order

ABSTRACT

Techniques are described for use in database and data management systems to efficiently capture and replicate changes that occur in database tables. The changes are captured in a transaction-consistent order, and in a format that may easily be searched and accessed using common relational queries.

CLAIM OF PRIORITY

This application claims priority under 35 USC § 120 to U.S. patentapplication Ser. No. 15/944,429, filed on Apr. 3, 2018, the entirecontents of which are hereby incorporated by reference.

TECHNICAL FIELD

This description relates to change capture for database tables.

BACKGROUND

Databases are generally used to store large quantities of information,in an organized manner that enables fast and convenient searching andother database operations. Often, it is necessary or desirable tocapture, replicate, or audit changes that occur in a particular databasetable.

For example, it may be desirable to replicate changes over time, forpurposes of reliability and availability of the data. In particular,replicating data (and associated changes) at a particular source tableto one or more target tables at disparate locations makes the data morewidely available for potential users, and less susceptible to aparticular system failure. In other examples, data change capture may beused to integrate data from multiple sites, for data warehousing andreporting, or for performing audits of database transactions.

Existing techniques for capturing database changes in the above andsimilar scenarios suffer from various drawbacks. In general, existingtechniques require excessive overhead in terms of latency, computationalresources, and human resources required for configuration andmaintenance. Further, the existing techniques may have limitedapplicability in heterogeneous environments, such as when change data isrequired to be replicated across different database systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system for database change capture withtransaction-consistent order.

FIG. 2 is a flowchart illustrating example operations of the system forFIG. 1.

FIG. 3 is a block diagram illustrating a first example implementation ofa replication log table of the system of FIG. 1.

FIG. 4 is a block diagram of a second example implementation of thereplication log table of FIG. 1.

FIG. 5 is a block diagram of a third example implementation of thereplication log table of FIG. 1.

FIG. 6 is a flowchart illustrating example change extraction techniquesused in the system of FIG. 1.

FIG. 7 is a flowchart illustrating entry insertion techniques for thereplication log table of FIG. 1.

FIG. 8 is a flowchart illustrating a timing of updates to thereplication log table of FIG. 1, in the context of an individualtransaction commit.

FIG. 9 is a timing diagram illustrating a relative timing of operationsof the examples of FIGS. 6-8.

FIG. 10 is a flowchart illustrating an example scenario for data changeextraction and associated generation of an entry for the replication logtable of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a system 100 for database change capturewith transaction-consistent order. In FIG. 1, a replication log table(RLT) manager 102 is configured to be implemented in the context of adatabase system 104 to capture database changes occurring within aparticular database instance 106, in response to, for example, queriesor other requests received from a database client 108.

For example, a query received from the client 108 may be received by asession layer 110 of the database instance 106. The session layer 110may be configured, for example, to compile the received query, formulatean execution plan for the query within the database instance 106, andultimately provide a query response, if any, to the client 108.

A transaction manager 112 is configured to implement logistical aspectsof resulting interactions with a database 114, and, in particular, withindividual database tables 116, such as an example source table 118 fromwhich resulting, captured database changes will be replicated andpropagated. A query executor 120 may receive one or more queries fromthe session layer 110, along with any relevant instructions for theprocessing thereof that may be received from the transaction manager112.

In more detail, the transaction manager 112 may be configured to trackrunning and closed transactions, including specific transaction events,such as transaction commits or rollback operations. The transactionmanager 112 may also be configured to inform the database 114 withrespect to such transaction events, so that the database 114 may executeany associated actions.

Thus, the system 100 generally represents and illustrates simplifiedscenarios in which various clients, represented by the client 108, areconfigured to issue a plurality of queries and associated databasecommands to a single/same instance 106 of the database system 104. Thevarious queries and commands are received at the session layer 110,which, as referenced, may compile or otherwise prepare the variousreceived queries and commands for use by the transaction manager 112 andthe query executor 120. The transaction manager 112 and the queryexecutor 120 may thus be configured to access the database 114, andultimately provide the requested or desired results for each of theclient(s) 108.

Following successful completion of a requested database operation, acontrol flow to execute a transaction commit may be executed. Forexample, the client 108 may send a commit command to the databaseinstance 106, and the session layer 110 may again interpret or compilethe received command. In this case, the received command is atransaction control command, and not, e.g., an SQL statement.Accordingly, the transaction manager 112 may proceed to commit thepreviously-started transaction.

In general, it will be appreciated that the database system 104 shouldbe understood to represent virtually any database system for which itmay be desirable to capture changes occurring within a database table,such as the source table 118. Accordingly, the session layer 110, thetransaction manager 112, and the query executor 120 should be generallyunderstood to represent example components provided to illustrate andexplain relevant operations of the RLT manager 102. In practice, ofcourse, a particular type of database instance 106 of a given databasesystem 104 may utilize any suitable component, or combination ofcomponents, required to process the particular type of query or otherrequest received from the client 108.

As the query executor 120 executes received queries or other changeoperation requests received from the client 108 with respect to thesource table 118, it may be desirable to propagate the resulting changesto a subscriber 122. In general, the subscriber 122 should be understoodto represent virtually any entity that may be interested in receivingreplicated changes from the source table 118, such as the types ofentities referenced above. Depending on the nature of a given usagescenario, the subscriber 122 may receive the replicated, propagatedchanges and either a push or pull (i.e., active or passive) manner,and/or using real-time, on request, just in time, or batch processing,as described in detail, below, and/or as would be appreciated by one ofskill in the art.

In the example of FIG. 1, the query executor 120 is illustrated asexecuting a change operator 124 that is configured to execute somechange with respect to the source table 118. Thus the change operator124 should be generally understood to represent any operator associatedwith, e.g., an insert, update, delete, erase, or write operation thatmay be executed with respect to the source table 118. In many of theexamples that follow, the change operator 124 is described as a datamanipulation language (DML) operator, which may be used to execute awrite event in which new data is added to the source table 118.

In general, although operations of the database system 104 are veryfast, particularly considering the volume of data often stored withinthe database 114, operations of the query executor 120, includingimplementation of the change operator 124, are not instantaneous. Forexample, it may occur that multiple requests are received from theclient 108, or from a plurality of additional clients (not pictured).Accordingly, it may occur that the source table 118 is locked for accessat a particular point in time. More generally, it may simply occur thatthe query executor 120 is tasked with executing a plurality of queriesand other operations against the database 114, thereby causing thechange operator 124 to wait for access to the source table 118.

Even once the change operator 124 begins executing with respect to thesource table 118, a corresponding quantity of time will be required tocomplete the requested change operations. For example, as describedherein, a particular transaction may include one or more queries,operations, statements, or other aspects, many of which are implementedin serial fashion. For example, an input to one statement may requireoutput from a preceding statement. In other examples, there may be otherreasons why a particular operation is forced to wait for required datato become available before proceeding. In order to facilitate operationsof the change operator 124 over time in these and other scenarios, arecord buffer 126 is configured to temporarily store change-relevantdata for operations of the change operator 124, during a time periodthat the change operator 124 may require access thereto in order toprocess the query and other aspects of the transaction requested by theclient 108.

As described in detail below, the RLT manager 102 includes a requesthandler 128 that is configured to determine a current requirement tocapture changes associated with the change operator 124. A built-inchange extractor 130 is configured to access the record buffer 126, inorder to extract required change data therefrom. An entry generator 132is thus provided with data necessary to generate an entry within areplication log table (RLT) 134.

By performing these and related operations across a plurality of datachanges, queries, and associated transactions, the replication log table134 serves to capture, replicate, and ultimately propagate changes tothe source table 118, in a manner which maintains a consistency of theorder in which the associated transactions occurred. Moreover, as alsodescribed below, the replication log table 134 may be generated as arelational table that may easily be queried by an otherwise-standardrelational interface 135 of the subscriber 122.

For example, as described below with respect to FIGS. 3-5, thereplication log table 134 may be accessed through the use of a standardSQL request and/or associated interface. In the simplified example ofFIG. 1, the relational interface 135 is illustrated as being implementedat the subscriber 122. However, it will be appreciated that thereplication log table 134 may also be accessed by a suitable interfaceimplemented at the database system 104, itself, or at the client 108.

In practice, the request handler 128 may initially be configured todetermine whether and when to initiate further operations of the RLTmanager 102. For example, the request handler 128 may determine whetherthe source table 118 is associated with a requirement for data changecapture, along with specific requirements of such data change capture.For example, the request handler 128 may determine a manner in which thereplication log table 134 is constructed with respect to a particularsource table or type of source table, examples of which are providedbelow with respect to FIGS. 3-5.

Based on the type or aspects of the replication log table 134 beingconstructed in the particular instance of the RLT manager 102, asdetermined by the request handler 128, the change extractor 130 mayproceed to extract the corresponding type of desired changes from therecord buffer 126, while the record buffer exists in the context of thechange operator 124 accessing the source table 118. In other words, asreferenced above, and described in detail below, the record buffer 126represents a temporary data store that exists while necessary for thechange operator 124 to access the stored data. During this time windowthat the record buffer exists, the change extractor 130 is configured toinitiate and implement extraction of relevant change data therefrom.

Using the extracted change data, the entry generator 132 may proceed toconstruct the specific type of the replication log table 134 specifiedby the request handler 128. Once generated, an entry may be temporarilystored within an RLT queue 136, in conjunction with incrementing a countof an RLT counter 138. In this way, as also described in detail below,the change operator 124 (along with, e.g., other change operators of agiven transaction) may continue its operations uninterrupted, even whencorresponding, resulting RLT entries are not able to be storedimmediately within the replication log table 134.

Then, as the entries of the RLT queue 136 are stored within thereplication log table 134, the entry generator 132 may decrement thecount of the RLT counter 138. Accordingly, the RLT counter 138 may reacha value of 0 once the RLT queue 136 has been emptied of RLT entries. Inthis way, the RLT manager 102 easily and quickly determines a time ofcompletion of data change capture required for the change operator 124within the replication log table 134.

Consequently, any operations of the database instance 106 that arerequired to wait for completion of operations of the RLT manager 102 maybe notified to proceed immediately upon such completion. In particular,for example and as described herein, completion of operations of the RLTmanager 102 with respect to the change operator 124 may trigger acorresponding transaction commit by the transaction manager 112 withrespect to the source table 118.

In this regard, the change extractor 130 and the entry generator 132 maybe configured to determine and include a transaction commit identifier,also referred to herein as a transaction (CID), which identifies andspecifies a each committed transaction, and a relative order of eachcommitted transaction (e.g., relative to preceding and subsequenttransactions). For example, each transaction CID may be stored within acorresponding entry of the replication log table 134. In this way, forexample, it is possible for the subscriber 122 to utilize the relationalinterface 135 to query the replication log table 134 for all entrieshaving a specified, desired transaction commit characteristic. Forexample, the subscriber 122 may query the replication log table 134 forall relevant transaction commits that occurred before or after aspecified time, or within a specified time range, or transaction range.

In the example of FIG. 1, the database system 104 is illustrated asbeing implemented using at least one computing device 140. As shown, theat least one computing device 140 is illustrated as including at leastone processor 142, along with a non-transitory computer readable storagemedium 144. Of course, the at least one computing device 140 should beunderstood to represent one or more computing device of any typesuitable to implement the database system 104.

Similarly, the at least one processor 142 should be understood torepresent two or more processors executing in parallel to implement thedatabase system 104. Thus, it will be further appreciated that thedatabase system 104 may itself be understood to be implemented in adistributed fashion. Nonetheless, as described in detail below,implementation of the RLT manager 102 within the same database instance106 as the source table 118 enables a fast and efficient transactioncommit process. For example, inclusion of the RLT manager 102 within thedatabase instance 106, and co-located with the source table 118, avoidsexpensive multi-system synchronist commit overhead cost (such as thoseassociated with the known 2-phase operation).

Finally with respect to FIG. 1, the non-transitory computer readablestorage medium 144 should be similarly understood to represent two ormore computer memories, possibly of multiple memory types, that may benecessary or helpful in implementing the database system 104. Forexample, the computer readable storage medium 144 may be utilized tostore instructions that are executed by the at least one processor 142to implement the RLT manager 102, and/or the database system 104, as awhole. Further, the computer readable storage medium 144 should beunderstood to represent any computer memory suitable to store the recordbuffer 126, or the replication log table 134.

In various implementations, some of which are referenced below, thedatabase system 104 may represent an in-memory or main memory databasesystem, in which the database 114 is stored using a main memory of theat least one computing device 140. Such in-memory database systemstypically use a secondary type of memory for more stable, longer termdata storage, such as a memory using disk-based storage. Accordingly,the computer readable storage medium 144 should be understood torepresent any of these or related types of computer memory that may beutilized to implement the database system 104.

FIG. 2 is a flowchart 200 illustrating example operations of the system100 of FIG. 1. In the example of FIG. 2, operations 202-210 areillustrated as separate, sequential operations. In variousimplementations, additional or alternative operations or sub-operationsmay be included, and/or one or more operation or sub-operation may beomitted. In these and other implementations, it may occur that any twoor more operations or sub-operations occur in a partially or completelyoverlapping or parallel manner, or in a nested, iterative, branched, orlooped fashion.

In the example of FIG. 2, a source table of a database instance isdetermined to require transaction-consistent change capture within areplication log table storing a change history of transactions for thesource table (202). For example, the request handler 128 of FIG. 1 maybe configured to determine that the source table 118 requires suchtransaction-consistent change capture within the replication log table134. For example, the request handler 128 may have access to a databaseor other storage specifying a requirement of the client 108, thesubscriber 122, and administrator of the database system 104, or anyother relevant, authorized entity that may request or require operationsof the RLT manager 102 with respect to change operations associated withthe source table 118. In other example implementations, the source table118 may be associated with a particular table type, or type orcharacteristic of data, which may be specified with respect to therequest handler 128 as requiring transaction-consistent change capture.

A first database change operation of a first transaction of thetransactions may be detected, the first database change operationwriting first record data to the source table using a record buffer thattemporarily stores the first record data during the first databasechange operation (204). For example, during the change operation of thechange operator 124, a corresponding write operation may proceed towrite first record data to the source table 118, using the record buffer126.

The first record data may be extracted from the record buffer (206). Forexample, the change extractor 134, in response to the correspondinginstruction from the request handler 128, may be configured to monitorcontents of the record buffer 126. The change extractor may thus beenabled to obtain the first record data from the record buffer 126.

Using the extracted first record data, a first replication log entry maybe generated that includes first metadata characterizing the firstrecord data, including a corresponding first transaction commitidentifier (CID) for the first transaction (208). For example, the entrygenerator 132 may generate an entry for the replication log table 134 asdescribed in detail below with respect to FIGS. 3-5, where the generatedentry may include the actual content of the record data extracted fromthe record buffer 126, along with metadata characterizing the firstrecord data and its associated transaction. In other implementations,the generated entry need not include the actual record data, and mayinstead include only the required metadata, along with a pointer orother identification of the actual record data within the source table118 itself.

The first replication log entry may be stored within the replication logtable, together with a second replication log entry that includes secondmetadata characterizing second record data of a second database changeoperation of a second transaction of the transactions, and including asecond transaction CID that identifies the second transaction asoccurring subsequently to the first transaction (210). For example, theentry generator 132 may be configured to store the generated entryinitially within the RLT queue 136, and ultimately inserted within thereplication log table 134. By including the first RLT entry togetherwith the first transaction CID and the second RLT entry together withthe second transaction CID, the replication log table 134 provides therequired transaction-consistent change capture characterizing a changehistory of transactions with respect to the source table.

FIG. 3 is a block diagram illustrating a first example implementation ofthe system 100 of FIG. 1. In the example of FIG. 3, the RLT subscriber122 accesses the database instance 106, as already described withrespect to FIG. 1.

In FIG. 3, a source table 302 and a source table 304 generally representthe examples of the source table 118 of FIG. 1, while a replication logtable 306 represents a particular example implementation of thereplication log table 134 of FIG. 1.

In the RLT 306, meta information 308 is included, which also includesthe transaction CID for each stored transaction. Further, a changed dataimage 310 of the actual data that has been changed as a result of eachcorresponding change operation is also included within the RLT 306.

In FIG. 3, the meta information 308 may include one or more of thefollowing columns of information characterizing a correspondingtransaction. For example, the meta information 308 may include atransaction ID to identify the transaction responsible for the relevantchange operation, as well as a statement ID denoting which statementwithin the transaction made the reference change. The meta information308 also may include a table ID or table name that indicates whichsource table has been affected by the change operation. A column foroperation type also may be included. For example, the operation type maybe indicated as one of insert, update, delete, or erase. In some cases,the operation type may relate to a data definition language (DDL) ortable truncate operation, which may, for example, have a schema-levelchange to the source table(s). In these cases, instead of performingrecord-level physical materialization, a single logical entry may begenerated and inserted into the RLT 306.

The meta information 308 also may include an old row ID, indicatingwhich source record has been deleted by the change in question, wherethe old row ID would have a null value in the case of an insertoperation. A new row ID may indicate which source record has been addedby the change, and would have a null value for a delete operation.Finally, as already described, the meta information 308 may also includethe transaction CID.

The example of FIG. 3 may be referred to as a full materializationimplementation, in which an entire change image and required metainformation are materialized and stored into the RLT 306. For example,as described with respect to FIG. 1, the change extractor 130 may beconfigured to extract the change data from the record buffer 126, andthe entry generator 132 may be configured to copy or otherwisematerialize the change data, along with the meta information 308, intothe replication log table 134/306.

Accordingly, the fully-materialized RLT 306 requires sufficient memoryresources to store both the meta information and an entirety of thechanged data image 310. Further, the fully-materialized RLT willgenerally require a corresponding amount of processing resourcesrequired to implement the change extractor 130 and the entry generator132 with respect to fully populating the RLT 306. Further, since eachsource table record copied from the source tables 306, 308 may havedifferences, e.g., and record format or length, the data image copiedfor inclusion within the RLT 306 may need to be encoded into avariable-length binary image, which may require an image interpreter tobe used at the RLT subscriber 122.

Advantageously, in the example of FIG. 3, the fully-materialized RLT 306provides a single access point for the RLT subscriber 122. For example,the RLT subscriber may obtain any desired change data through the use ofa simple relational query, such as a relational query 312 (illustratedas “Select*from RLT where CID> . . . ” in FIG. 3.

FIG. 4 is a block diagram illustrating a second example implementationof the system 100 of FIG. 1. In the example of FIG. 4, an RLT 402 isonly partially materialized from the change data captured by the changeextractor 130.

That is, as shown, the RLT 402 includes the meta information 404materialized by the change extractor 130 and the entry generator 132,without requiring materialization of the changed image of the changedata itself. Instead, as shown, the changed image may be indirectlyaccessed, using the source tables 302, 304. For example, record IDinformation stored within the meta information 404 of the RLT 402 may beused to store corresponding, individual records 406 of the source tables302, 304.

Thus, in order to access desired change data, the subscriber 122 maysimply use the relational interface 135 to issue a query 408, shown inFIG. 4 as “Select t1:t2*from RLT as t1 join < are table> as 12 onRLT.RiD t2.RiD where t1.CID> . . . ,”. As shown, the RLT subscriber 122may retrieve any desired change history simply by joining the RLT 402and necessary, corresponding source table 302 or 304, as shown in thequery 408.

FIG. 5 is a block diagram illustrating a third example implementation ofthe system 100 of FIG. 1. In the example of FIG. 5, an implementation ofan RLT 502 is illustrated, in which meta information 504 again includesthe transaction CID, as illustrated. As further illustrated, withrespect to source tables 506, 508, a column 510 for the transaction CIDis also included.

In response to operations such as delete, update, or DDL, the RLT 502 isupdated with corresponding, generated entries, using an appropriatequery generated by the RLT subscriber 122 that leverages theavailability of the CID column 510. However, in the example, it is notnecessary to notify/update the RLT directly in response to an insertoperation, or to capture changes associated with an insert operation inthe RLT. Instead, the new data resulting from such an insert operationwill simply reside within the appropriate source table, and the RLTsubscriber 122 may simply obtain the corresponding inserted data at atime of querying that source table.

In other words, for example, when the RLT subscriber 122 retrievesdesired change history via the RLT 502, the corresponding record may beretrieved by accessing the RLT 502 to find the inserted record byquerying the CI in the column 510. Thus by additionally maintaining theCID column 510 for each source table, an inserted record may be directlyretrieved from the source tables, using the query 512, in which arecord(s) with a CID value larger than a last-retrieved CID value isobtained, and the newly-inserted record is identified from the relevantsource table.

Accordingly, of the three example implementations of FIGS. 3, 4, and 5,the example RLT 502 of FIG. 5 will generally consume the least amount ofmemory space, as well as minimizing an amount of processing resources toexecute copy operations to the memory space. On the other hand, in somescenarios, it may be undesirable or difficult to add the CID column 510to an existing source table schema.

The partial materialization option of the example of FIG. 4 alsorequires fewer memory/processing resources then the example of FIG. 3.However, for both of the examples of FIGS. 4 and 5, the subscriber 122(representing multiple subscribers) may incur potential contention withonline source transactions, due to the need to access the various sourcetables, themselves.

Meanwhile, the example implementation of FIG. 3 provides the highestconsumption of resources, but is likely to provide the fastest and mostdirect access to desired change data, as experienced by the RLTsubscriber 122. Thus, selection between one or more of the exampleimplementations of FIGS. 3-5, or variations thereof, should beunderstood to require a suitable design choice on the part of anadministrator or other designer of the system 100 of FIG. 1, and takinginto account a particular use context of the implementation beingdesigned.

FIG. 6 is a flowchart illustrating example operations of the changeextractor 130 and the entry generator 132 of FIG. 1. FIG. 6 illustratesoperations 602-610, which are example operations associated with a callpath performed during a typical, example write operation, such as a DMLquery. In the example, such a DML query is received (602), whereupon thereceived query may be compiled (604), such as at the session layer 110.

The DML may then be processed (606), such as by the query executor 120,using the change operator 124 and the record buffer 126 of FIG. 1.Further in typical, example implementations, a redo log may be generated(608), which is used in case of system failure. The process flow maythen return (610), so as, for example, to receive a second DML.

In conjunction with the DML processing (606), the change extractor 130and entry generator 132 may proceed to extract required changes andgenerate a corresponding RLT entry (612). The entry generator 132 mayfurther be configured to increment the RLT counter 138 by one (614) andin queue the RLT entry into the RLT queue 136 (616).

Thus, FIG. 6 illustrates that the described change extraction and entrygeneration operations are called at a time of DML processing (606). TheRLT counter 138 may be implemented as a per-transaction counter, whichmay be initialized when the relevant transaction begins.

It will be appreciated with respect to the three example implementationsof FIGS. 3, 4, and 5 that, depending on a nature of the RLT beingimplemented, contents of the captured changes may vary, whilemaintaining the same or similar execution path illustrated with respectto FIGS. 6-10. For the example of FIG. 5, operations 612-616 may not berequired for source table insertion operations, as described above withrespect to FIG. 5.

FIG. 7 is a flowchart illustrating an example insert operation for thereplication log table 134 of FIG. 1, or any of the RLTs 306, 402, 502 ofFIGS. 3, 4, 5, respectively. As shown, the entry generator 132 mayproceed by de-queuing a particular entry from the RLT queue 136 (702).The de-queued entry may then be inserted into the RLT 134 (704), and theRLT counter 138 may be decremented.

FIG. 8 is a flowchart illustrating an example transaction commitoperation in accordance with the system 100 of FIG. 1. In the example ofFIG. 8, the following receipt of a commit request (802), the pendingredo log entries may be flushed (804). The transaction may then commit(806), and the overall process flow may return (808) to proceed with asubsequent transaction. As shown in FIG. 8, following the commit requestbeing received (802), the process flow waits until the RLT counter 138has decremented to the value of 0 (810), before proceeding to flush thepending redo log entries (804).

FIG. 9 is a timing diagram illustrating a timing of composite operationsof the flowcharts of FIGS. 6-8. In the example of FIG. 9, a first DML902 is received from the client and undergoes execution 904 in aforeground server process. A corresponding RLT entry 906 is extracted,generated, en-queued/de-queued, and counted in a background serverprocess, and in conjunction with the described operations of FIGS. 6 and7.

Similarly, a second DML 908 may be received and executed 910 in aforeground server process. The corresponding RLT entry 912 may thus behandled for the DML 908, as just described with respect to the RLT entry906.

As described with respect to FIG. 8, upon receipt of a transactioncommit request 914, it may be necessary to wait (916) in the event thatany unfinished RLT entry remains. As also described with respect to FIG.8, once the RLT counter 138 reaches a value of 0, the transaction commitexecution 918 may proceed in the foreground server process.

For example, the CID itself may be determined at a time of transactioncommit, which is the very last step of a transaction execution. Toassociate the determined CID value into already-generated record imagesof the transaction, the following mechanism may be used. For example, anappropriate object may be created for record versions created by asingle/same transaction. The object may be configured to store the CIDvalue of the transaction, and also to link to the record versionscreated by the transaction. Before the transaction commits, the CIDvalue may be set as null, and when the CID value of the transaction isdetermined, the value may be written to the object in an atomic way. TheCID value may then be copied back to the record versions asynchronouslyin a background process. From the perspective of record versions, thisasynchronous copy operation of CID values may be used in conjunctionwith a link to the corresponding object. When the CID value of a recordversion is needed and the CID value itself is not yet copied to therecord version, the value can be retrieved using the link. In themulti-version concurrency control (MVCC) storage architecture of SAPHANA, an example of such an object is implemented as the “TransContext”object.

It is possible that an RLT-involved transaction may encounter a rollbackevent, such as when a multi-statement change operation encounters anerror prior to a final statement (and transaction commit) being reached.Then, in addition to the changes at the source tables, the changes atthe corresponding RLT table may be rolled back. Any pending entry in theRLT queue for the transaction may be deleted from the corresponding RLTqueue, and any ongoing RLT operation for the transaction may becancelled.

FIG. 10 is a flowchart illustrating more detailed exampleimplementations of the change extractor 130 and the entry generator 132of FIG. 1, such as may be executed during the extraction/generationoperation 612 of FIG. 6. Thus, for example, the operations 1002, 1004,1006 may be understood to be implemented, e.g., within the backgroundserver RLT processing 906 or 912 of FIG. 9.

FIGS. 8 and 9 illustrate a model of optimistic synchronous changepropagation, in which a write operation (e.g., DML operation) proceedswithout waiting for captured changes to be copied to the relevant RLT,but the transaction commit waits for all pending RLT updates to becompleted. In this way, consistency between source table changes andcorresponding RLT entries are assured, without adding delays toindividual write/DML operations. Even if a commit operation delayoccurs, such a wait incurred by RLT background processing will generallybe relatively minimal (and/or may be hidden) in many multi-statement andclient-server architectures, such as may be used to implement the system100 of FIG. 1.

In the example of FIG. 10, a received DML may be processed, includingreading newly-added values from the record buffer 126 (1002). If theread value does not need a previously-designated RLT condition (1004),then the process may end (1005). Otherwise, a new, corresponding RLTentry may be generated, including reading the new values directly fromthe record buffer 126 (1006).

Thus, in the examples of FIGS. 1-10, techniques are described for usingan in-database, built-in change extractor, such as the change extractor130, which may be implemented as an extension of a DML operator, such asthe change operator 124. Captured changes may thus be copied and storedto a system-internal table, such as the various RLT implementationsdescribed herein, representing a queryable, relational table.Accordingly, an external application program (represented by RLTsubscriber 122) can conveniently retrieve the change history, e.g., byusing plain SQL interfaces. In addition, to enable any externalapplication program reading the change history in atransaction-consistent order from RLT, each record in RLT stores the CIDof the transaction that changed the record.

Further, in the various examples described herein, the various RLTimplementations are co-located with the same database instance thatincludes the source tables. Accordingly, expensive multi-systemsynchronous commit overhead costs may be avoided (such as the expensivetwo-phase commit operation referenced above). Even when the sourcesystem is a distributed database system, in which tables may bedistributed to multiple database instances, each database instance maybe configured to maintain its own local RLT instance for source tableslocated in that database instance. It is also possible to maintainmultiple RLT instances within a single database instance, e.g., to avoida single point of contention for RLT subscriber access.

By way of more specific example of RLT subscriber scenarios, the RLTsubscriber 122 may periodically check for (and retrieve) any new recordin the RLT 134. In order to ensure transactional ordering of processedRLT entries, one method is simply to use a single subscriber per sourcetable. For further parallelism, it is also possible to assign a separatesubscriber per table. When there is only one dedicated subscriber, anRLT truncation can be driven by the sole subscriber such that the RLTsubscriber 122 deletes the processed RLT records using SQL statements.When there are multiple subscribers, an additional read counter may needto be added to the RLT structure. In such cases, each subscriberincrements the read counter on its processing and then, if the readcounter reaches a certain value, a background operation can be used todelete the corresponding record.

The described techniques improve upon existing techniques fortransaction-consistent change-ordered database capture. For example,some existing techniques use an SQL trigger at a receiving application,for each relevant DML operation and source table. These techniquesinvolve performance overhead, as well as imposing a burden onapplication programmers to write and maintain the SQL triggers. Further,the SQL trigger may need to reconstruct a record image when updating atarget table for replicated changes, using parameter values obtainedthrough the trigger code, which also imposes additional overhead.

Although in-database replication methods exist, existing versions aregenerally only operable within a single (or multiple homogeneous)database system(s). Further, redo logs, such as referenced above, may beread in FIG. 6 at operation 608, may be read, and the results may beconverted to a sequence of SQL statements. However, a requiredconversion logic may be heavily dependent on a format of the particulartype of redo log being used, thereby adding a burden of creating andmaintaining consistency/compatibility between the conversion logic andthe particular redo log format. Further, the conversion logic mayrequire an additional disk operation to read from the redo log, therebylengthening a time required for replication propagation.

Implementations of the various techniques described herein may beimplemented in digital electronic circuitry, or in computer hardware,firmware, software, or in combinations of them. Implementations may beimplemented as a computer program product, i.e., a computer programtangibly embodied in an information carrier, e.g., in a machine-readablestorage device, for execution by, or to control the operation of, dataprocessing apparatus, e.g., a programmable processor, a computer, ormultiple computers. A computer program, such as the computer program(s)described above, can be written in any form of programming language,including compiled or interpreted languages, and can be deployed in anyform, including as a stand-alone program or as a module, component,subroutine, or other unit suitable for use in a computing environment. Acomputer program can be deployed to be executed on one computer or onmultiple computers at one site or distributed across multiple sites andinterconnected by a communication network.

Method steps may be performed by one or more programmable processorsexecuting a computer program to perform functions by operating on inputdata and generating output. Method steps also may be performed by, andan apparatus may be implemented as, special purpose logic circuitry,e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. Elements of a computer may include atleast one processor for executing instructions and one or more memorydevices for storing instructions and data. Generally, a computer alsomay include, or be operatively coupled to receive data from or transferdata to, or both, one or more mass storage devices for storing data,e.g., magnetic, magneto-optical disks, or optical disks. Informationcarriers suitable for embodying computer program instructions and datainclude all forms of non-volatile memory, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor andthe memory may be supplemented by, or incorporated in special purposelogic circuitry.

To provide for interaction with a user, implementations may beimplemented on a computer having a display device, e.g., a cathode raytube (CRT) or liquid crystal display (LCD) monitor, for displayinginformation to the user and a keyboard and a pointing device, e.g., amouse or a trackball, by which the user can provide input to thecomputer. Other kinds of devices can be used to provide for interactionwith a user as well; for example, feedback provided to the user can beany form of sensory feedback, e.g., visual feedback, auditory feedback,or tactile feedback; and input from the user can be received in anyform, including acoustic, speech, or tactile input.

Implementations may be implemented in a computing system that includes aback-end component, e.g., as a data server, or that includes amiddleware component, e.g., an application server, or that includes afront-end component, e.g., a client computer having a graphical userinterface or a Web browser through which a user can interact with animplementation, or any combination of such back-end, middleware, orfront-end components. Components may be interconnected by any form ormedian of digital data communication, e.g., a communication network.Examples of communication networks include a local area network (LAN)and a wide area network (WAN), e.g., the Internet.

While certain features of the described implementations have beenillustrated as described herein, many modifications, substitutions,changes and equivalents will now occur to those skilled in the art. Itis, therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the scope of theembodiments.

What is claimed is:
 1. A computer program product, the computer programproduct being tangibly embodied on a non-transitory computer-readablestorage medium and comprising instructions that, when executed, areconfigured to cause at least one computing device to: detect a firstdatabase change operation of a first transaction of the transactions,the first database change operation writing first record data to asource table of a database instance using a record buffer thattemporarily stores the first record data during the first databasechange operation; extract the first record data from the record buffer;generate, using the extracted first record data, a first replication logentry that includes first metadata characterizing the first record data,including a corresponding first transaction commit identifier (CID) forthe first transaction; and store the first replication log entry withina replication log table storing a change history of transactions for thesource table.
 2. The computer program product of claim 1, wherein theinstructions, when executed, are further configured to cause the atleast one computing device to: store the first replication log entrywithin the replication log table, together with a second replication logentry that includes second metadata characterizing second record data ofa second database change operation of a second transaction of thetransactions, and including a second transaction CID that identifies thesecond transaction as occurring subsequently to the first transaction.3. The computer program product of claim 1, wherein the instructions,when executed, are further configured to generate the first replicationlog entry, including causing the at least one computing device to:generate the first replication log entry including the first record dataand the first metadata.
 4. The computer program product of claim 3,wherein the instructions, when executed, are further configured to causethe at least one computing device to: provide the first replication logentry to a subscriber to the replication log table for the source table,and based on a relational query specifying the first replication logentry.
 5. The computer program product of claim 1, wherein theinstructions, when executed, are further configured to generate thefirst replication log entry, including causing the at least onecomputing device to: generate the first replication log entry includingthe first metadata; and store the first replication log entry in thereplication log table, without copying the first record data to thereplication log table.
 6. The computer program product of claim 5,wherein the instructions, when executed, are further configured to causethe at least one computing device to: provide the first record data fromthe source table to a subscriber to the replication log table for thesource table, in response to a relational query and based on the firstreplication log entry, the first metadata, and the source table.
 7. Thecomputer program product of claim 1, wherein the instructions, whenexecuted, are further configured to generate the first replication logentry, including causing the at least one computing device to: generatethe first replication log entry including the first metadata; and storethe first transaction CID within a transaction CID column of the sourcetable.
 8. The computer program product of claim 7, wherein theinstructions, when executed, are further configured to cause the atleast one computing device to: provide the first record data from thesource table to a subscriber to the replication log table for the sourcetable, in response to a relational query and based on the firstreplication log entry, the first metadata, and the first transaction CIDwithin the transaction CID column of the source table.
 9. The computerprogram product of claim 1, wherein the instructions, when executed, arefurther configured to cause the at least one computing device to:generate a second replication log entry that includes second metadatacharacterizing second record data of a second database change operationof the first transaction of the transactions, without waiting forcompletion of storage of the first replication log entry; and wait forstorage of the first replication log entry and the second replicationlog entry to the replication log table before proceeding with atransaction commit of the first transaction of the transactions.
 10. Thecomputer program product of claim 1, wherein the source table and thereplication log table are both located within the database instance. 11.A computer-implemented method, comprising: detecting a first databasechange operation of a first transaction of the transactions, the firstdatabase change operation writing first record data to a source table ofa database instance using a record buffer that temporarily stores thefirst record data during the first database change operation; extractingthe first record data from the record buffer; generating, using theextracted first record data, a first replication log entry that includesfirst metadata characterizing the first record data, including acorresponding first transaction commit identifier (CID) for the firsttransaction; and storing the first replication log entry within areplication log table storing a change history of transactions for thesource table.
 12. The method of claim 11, further comprising: storingthe first replication log entry within the replication log table,together with a second replication log entry that includes secondmetadata characterizing second record data of a second database changeoperation of a second transaction of the transactions, and including asecond transaction CID that identifies the second transaction asoccurring subsequently to the first transaction.
 13. The method of claim11, comprising: generating the first replication log entry including thefirst record data and the first metadata; and providing the firstreplication log entry to a subscriber to the replication log table forthe source table, and based on a relational query specifying the firstreplication log entry.
 14. The method of claim 11, comprising:generating the first replication log entry including the first metadata;storing the first replication log entry in the replication log table,without copying the first record data to the replication log table; andproviding the first record data from the source table to a subscriber tothe replication log table for the source table, in response to arelational query and based on the first replication log entry, the firstmetadata, and the source table.
 15. The method of claim 11, comprising:generating the first replication log entry including the first metadatastoring the first transaction CID within a transaction CID column of thesource table; and providing the first record data from the source tableto a subscriber to the replication log table for the source table, inresponse to a relational query and based on the first replication logentry, the first metadata, and the first transaction CID within thetransaction CID column of the source table.
 16. The method of claim 11,comprising: generating a second replication log entry that includessecond metadata characterizing second record data of a second databasechange operation of the first transaction of the transactions, withoutwaiting for completion of storage of the first replication log entry;and waiting for storage of the first replication log entry and thesecond replication log entry to the replication log table beforeproceeding with a transaction commit of the first transaction of thetransactions.
 17. The method of claim 11, wherein the source table andthe replication log table are both located within the database instance.18. A system comprising: at least one processor; and a memorycommunicatively coupled to the at least one processor, the memorystoring instructions which, when executed, cause the at least oneprocessor to perform operations comprising: detecting a first databasechange operation of a first transaction of the transactions, the firstdatabase change operation writing first record data to a source table ofa database instance using a record buffer that temporarily stores thefirst record data during the first database change operation; extractingthe first record data from the record buffer; generating, using theextracted first record data, a first replication log entry that includesfirst metadata characterizing the first record data, including acorresponding first transaction commit identifier (CID) for the firsttransaction; and storing the first replication log entry within areplication log table storing a change history of transactions for thesource table.
 19. The system of claim 18, wherein the instructions, whenexecuted, cause the at least one processor to perform operationscomprising: storing the first replication log entry within thereplication log table, together with a second replication log entry thatincludes second metadata characterizing second record data of a seconddatabase change operation of a second transaction of the transactions,and including a second transaction CID that identifies the secondtransaction as occurring subsequently to the first transaction.
 20. Thesystem of claim 18, wherein the instructions, when executed, cause theat least one processor to perform operations comprising: generating thefirst replication log entry including the first record data and thefirst metadata; and providing the first replication log entry to asubscriber to the replication log table for the source table, and basedon a relational query specifying the first replication log entry.